4-digit decimal counter

Build a 4-digit BCD (binary-coded decimal) counter. Each decimal digit occupies one nibble of q:

• q[3:0] — ones digit
• q[7:4] — tens digit
• q[11:8] — hundreds digit
• q[15:12] — thousands digit

The ones digit increments every clock cycle; each upper digit increments only when all digits below it wrap. Also output the increment enables for the upper three digits, derived combinationally from the current count:

• ena[1] = 1 when the ones digit equals 9
• ena[2] = 1 when the ones and tens digits both equal 9
• ena[3] = 1 when the ones, tens, and hundreds digits all equal 9

reset is synchronous, active-high, and clears the count to 0000.